Nonlinear optical devices, such as frequency mixers and optical parametric converters, are widely used for generating wavelengths that cannot be generated efficiently with currently available lasers. Examples of commonly-used frequency mixers are second harmonic generators (SHGs), third harmonic generators (THGs), and fourth harmonic generators (FHGs). As an example, a SHG may receive a fundamental optical radiation signal having a wavelength of 1.06 micrometers. Through a process that occurs within a nonlinear medium, a portion of the fundamental is converted to a signal at half the wavelength, 532 nm, or twice the frequency, of the fundamental.
Also of interest are optical parametric devices, both oscillators and amplifiers, that generate continuously-tunable radiation when pumped by fixed-wavelength lasers or their harmonics.
For most applications, the efficiency of these converters should be high, preferably near the quantum limit. This is especially important when several of these devices are used in cascade, as in the case of optical parametric converters pumped by the third or fourth harmonic of, by example, a Nd:YAG laser.
The above mentioned nonlinear processes are typically represented in terms of photon energies or frequencies (o) of the interacting waves, in accordance with the following two processes: EQU Frequency mixing o.sub.1 +o.sub.2 .fwdarw.o.sub.3 and (1) EQU Parametric conversion o.sub.3 .fwdarw.o.sub.1 +o.sub.2 ( 2)
SHG is a special case of process (1), with o.sub.1 =o.sub.2. THG is a cascade of two mixing processes, i.e. a SHG followed by a mixer with o.sub.1 =o and o.sub.2 =2o. FHG is a cascade of two SHG processes, i.e. o+o.fwdarw.2o, followed by 2o+2o.fwdarw.4o.
Two important parameters characterizing the performance of wavelength converters are the conversion efficiency and the optical energy density, or fluence, on the nonlinear medium and the optical components of the converter system. For pulsed laser sources, the conversion efficiency is specified as an energy or a photon conversion efficiency.
The energy conversion efficiency is defined as the ratio of the energy in the output pulse(s) and the energy in the input pulse(s). The maximum value of the energy conversion efficiency is 100 percent for process (1), while for process (2) the energy conversion efficiency is equal to o.sub.1 /o.sub.3 or o.sub.2 /o.sub.3 for the generation of o.sub.1 or o.sub.2, respectively.
The photon conversion efficiency is defined as, for process (1), the ratio of the number of photons in the output pulse and the (equal) number of photons in each of the input pulses. For process (2) the photon conversion efficiency is defined as the ratio of the equal number of photons in each of the generated pulses, and the number of photons in the input pulse. For o.sub.1 .noteq.o.sub.2, the maximum photon conversion efficiency is 100 percent. For the degenerate case with o.sub.1 =o.sub.2, the maximum photon conversion efficiency is 50 percent for process (1), and 200 percent for process (2).
For practical applications it is desirable to maximize the conversion efficiency while minimizing the fluences (optical power densities). In particular, the fluences (optical power density) should be maintained below a value at which irreversible damage occurs either to the nonlinear medium or to optical components of the wavelength conversion system. State-of-the-art wavelength converters have conversion efficiencies well below the theoretical maximum values. For example, SHGs used with pulsed (Q-switched) Nd:YAG lasers have typical energy conversion efficiencies of 50 percent, while THGs and FHGs have energy conversion efficiencies of approximately 30 percent and 20 percent, respectively. Optical parametric converters, both oscillators (OPOs) and amplifiers (OPAs), have typical photon conversion efficiencies below 50 percent.
Techniques for increasing the efficiency of frequency mixers and optical parametric converters pumped by low power, usually continuous-wave (CW) lasers, are described by A. Ashkin, G. D. Boyd and J. M. Dziedzic: IEEE J. Quantum Electronics QE-2, p. 109-124 (1966); W. J. Kozlovsky, C. D. Nabors and R. L. Byer, IEEE J. Quantum Electronics QE-24, p. 913-919 (1988); and by W. J. Kozlovsky, W. Lenth, E. E. Latta, A. Moser and G. L. Bona, Appl. Phys. Lett. 56, p. 2291-2292 (1990). With these techniques, strong feedback is applied at the input wavelength, or the output wavelength, to increase the low efficiencies obtained with single-pass converters pumped by low power sources. The nonlinear medium is placed in a resonant cavity with mirror reflectivities close to 100% (typically 97-99%), and the cavity linear losses are maintained at very low value (typically &lt;1%). The beam size within the nonlinear medium is made very small, typically several tens to hundreds of microns. For the case of feedback at the input wavelength, efficiency increases of several orders of magnitude can be obtained over a no-feedback case.
What is not taught by this prior art, and what is thus one object of the invention to provide, is a technique to improve the conversion efficiency for high power, pulsed laser sources. As an example, the power of a Nd:YAG laser pulse, having energy of 0.01 Joules and a pulse length of 10 nanoseconds, is 10.sup.6 Watts, as compared to milliWatts to several tens of Watts for a typical CW laser. The present invention overcomes the limitations in conversion efficiency which are present with high power sources. These efficiency limitations result from the fact that the laser intensity is nonuniform in both space and time, or, stated differently, the laser beam profile is not "top-hat", and the laser pulse shape is not rectangular.
These nonuniformities limit the conversion efficiency of high power, pulsed wavelength converters, in two ways. First, even though conversion efficiencies approaching the quantum limit can be obtained in the most intense part(s) of the beam/pulse, the efficiencies are lower in the spatial and temporal "wings" of the beam/pulse. In principle, this limitation could be overcome by further increasing the intensity by reducing the beam size. However, in practice this often leads to intensities and/or fluences (optical power densities) within the high intensity part(s) of the beam/pulse which exceed the damage limit of the nonlinear material and/or the optical components.
A second limitation imposed by beam/pulse nonuniformities is that the mixing and parametric processes can be reversible. Non-degenerate mixing processes are reversible except for the practically unrealistic case where the beam profiles and pulse shapes of the two input fields are identical, and the ratio of their energies is equal to the ratio of the frequencies of the two fields. Parametric processes are reversible both in the degenerate and non-degenerate cases. As an example, for the case of THG, as the intensity of the o and 2o input beams is increased, back-conversion of energy at 3o to radiation at o and 2o occurs, thus preventing simultaneous high conversion across the entire beam/pulse. Similarly, in parametric converters, reverse energy transfer occurs from the generated o.sub.1 and o.sub.2 waves back to the o.sub.3 pump wave at sufficiently high input pump intensities.